Pseudohatschekiidae, a new family for Pseudohatschekia branchiostegi Yamaguti, 1939 (Crustacea: Copepoda: Siphonostomatoida) from Branchiostegus spp. (Teleostei: Perciformes: Malacanthidae) in Japanese waters, with descriptions of the early developmental stages of P. branchiostegi.

The siphonostomatoid copepod Pseudohatschekia branchiostegi Yamaguti, 1939 is redescribed in detail based on specimens of both sexes collected from the gill filaments of three species of Branchiostegus Rafinesque (Teleostei: Malacanthidae) captured in Japanese waters. Descriptions of two naupliar stages and the infective copepodid stage of P. branchiostegi are also provided for the first time, and P. mebaru Yamaguti, 1939 is recognised herein as a junior synonym of P. branchiostegi. The latter copepod species represents a new family, the Pseudohatschekiidae fam. nov., of the Siphonostomatoida characterised by the following apomorphies: (a) two free pedigerous somites present between the cephalothorax and the genital complex; (b) a chelate antenna bearing two digitate processes and a thin cuticular covering; (c) a unilobate maxillule, with the palp completely fused to the endite and represented by a surface seta; (d) a rounded process furnished with pectinate membranes on the apex of the maxillary basis; (e) 2-segmented rami on legs 1-3; and (f) the absence of leg 4. It is also now evident that P. branchiostegi is a relatively common and abundant parasite of Branchiostegus spp. in the Far East.